Axial piston pumps useful for hydraulic applications are well known in the art. These pumps are characterized by the presence of multiple pistons positioned axially with respect to each other. The axially-positioned pistons oscillate linearly in conjunction with sets of check valves, to pressurize fluid. In one family of axial piston pumps, the oscillating pistons are situated in a rotating drum and are in contact with a swash plate or wobbler disk that has a slanted face for imparting sliding piston motions. The check valves are generally in the form of a stationary disk having slots to serve as in-out fluid passages. In another family of axial-piston pumps, the multiple pistons are situated in a stationary cylinder while a rotating cam disk having a slanted face is in contact with the pistons to impart oscillating motions. In both cases, return springs are generally used to provide the piston return forces.
In rotating-cam pumps, separate inlet and outlet check valves in the form of balls and poppets are often used. U.S. Pat. No. 3,348,495 issued to Orshansky teaches a dual-cam axial-piston pump of this type. The outlet check valve of this type of pump is easy to manage, requiring a simple one-way valve at the bottom of each piston cavity. The inlet check valve of this family of pumps, on the other hand, is more difficult to configure. Orshansky discloses the use of another set of pistons purely for the valving purpose.
U.S. Pat. No. 4,776,260, issued to Vincze, discloses a cam-driven axial piston pump which utilizes ball check valves at the inlet and outlet of each piston cavity. A six-piston pump of this design, for instance, has six inlet ball check valves and six outlet ball check valves.
In any pump, the design of the check valves is an integral part of the pump design. A pump cannot function without good check valves. The reverse process of converting linear oscillatory motion of multiple, axially positioned pistons to the rotatory motion of a shaft is also very common. This is the essence of fluid-powered motors. In such motors, the potential energy stored in pressurized fluids is released by pushing a set of axially-positioned pistons to rotate a shaft through a cam disk having a slanted face. In some cases, the device capable of generating shaft power is also a pump. Orshansky teaches an axial-piston pump that can function as a motor simply by reversing the role of the fluid. The pump disclosed in Vincze is not reversible, and cannot function as a motor due to the check valves involved.
Reversible pump-motor devices are rather rare and their capability is not even in the two different functions. There are many other fluid-powered motors that are simpler and less expensive than axial-piston motors. Therefore, axial-piston motors must possess unique capabilities in order to be viable in the marketplace. This is also true for axial-piston pumps.